Coplanar Microwave Resonators Research Articles

Effects of disorder on the microwave properties ofMgB2polycrystalline films

The role of disorder in superconducting magnesium diboride $({\mathrm{MgB}}_{2})$ policrystalline films is investigated in the high frequency range by a coplanar microwave resonator technique. Two sources of disorder are considered, heavy-ion irradiation damage and sample ageing. Microwave measurements are analyzed in the framework of the two-gap model with strong interband scattering contribution. It turns out that disorder enhancement increases the interband scattering rate, resulting in a reduction of the surface resistance at low temperatures, due to a slight increase of the $\ensuremath{\pi}$ gap. Moreover, increasing disorder at grain boundaries induces a nonmonotonic residual surface resistance, showing the features of a resistive behavior for the highest disorder level. Finally, the effects of the different kinds of disorder on the intrinsic and on the grain-boundary properties of the ${\mathrm{MgB}}_{2}$ films are compared and discussed.

Calculation of the characteristics of coplanar resonators for kinetic inductance detectors

Photon detectors based on the change of kinetic inductance of a thin superconducting film have a number of applications, particularly in astronomy, owing to their high sensitivity and ease of integration into large arrays. Here we discuss in detail the analysis of kinetic inductance detectors that use thin film microwave coplanar resonators. Photon absorption decreases the electron pair density, increasing the magnetic penetration depth /spl lambda/, which causes a decrease in the resonant frequency (or phase) of an irradiated resonator. To quantify this effect, we first compute the resonator current distribution, from which the /spl lambda/-dependent parameters (such as kinetic inductance) are calculated. Optimum responsivity for phase measurement is achieved by using the thinnest film with the narrowest center conductor width at the lowest possible temperature. However, the responsivity is compromised by extrinsic microwave losses, in particular due to residual surface resistance, which is likely to be significant in the thinnest films.

Application of modulated optical reflectance method for high-temperature superconducting film characterization

The modulated optical reflectance (MOR) method is applied for nondestructive characterization of high-temperature superconducting (HTS) films and thin-film devices. An optimized optical scheme is developed employing He–Ne laser and diode laser of 780nm wavelength, and via a photodetector aligned to a lock-in amplifier, which is able to monitor the reflectance and technological inhomogeneity in the focal spot on the sample film. Microwave coplanar resonators patterned from YBa2Cu3O7−x (YBCO) films of 100nm thickness have been examined employing the MOR scheme. Unmodulated reflectance signals and MOR are compared between YBCO film and the substrate, MOR scans provide better indication of the structural differences due to the employed thermal modulation. A characteristic feature of YBCO films with embedded oxide crystals, referred to as assemblies of imperfections, could be detected. Automation of the scanning process and computerization of data acquisition are being developed.

Sputtering of YSZ buffer layers on ( [formula omitted])Si for use in YBa 2Cu 3O 7 based microwave circuits

We have produced yttria-stabilized zirconia (YSZ) films on Si( 1 0 0 ) for use as buffer layers for YBa 2Cu 3O 7 microwave devices. The YSZ films, 30–100 nm thick, were grown by low frequency pulsed dc and rf magnetron sputtering under a variety of experimental conditions. YBa 2Cu 3O 7 films, 300–600 nm thick, were grown using pulsed laser deposition in an atmosphere of 0.5 Torr O 2 at 780°C. The critical temperature ( T c( R=0)) of the films produced was 80 K. These were subsequently used to build microwave coplanar resonators. We designed a simple coplanar T resonator structure. Microwave tests at T=20 K showed a resonance with a quality factor in excess of 20000 at a frequency 3.87 GHz.

The coplanar resonator technique for determining the surface impedance of YBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films

We describe how coplanar microwave resonators fabricated from patterned thin films of YBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) (YBCO) can he used to measure the ab-plane microwave surface impedance Z/sub s/=R/sub s/+jX/sub s/ of the films, in particular the absolute value and temperature dependence of the magnetic penetration depth /spl lambda/. The current distribution of the resonator is calculated by modelling the resonator as a network of coupled transmission lines of rectangular cross-sections; this is then used to estimate the ab-plane /spl lambda/(T) from the measurements of resonators of different geometries patterned onto the same film. We obtain values of /spl lambda/(0) in the range 150-220 nm. The unloaded quality factors of the linear resonators at 7.95 GHz are around 45000 at 15 K and around 6500 at 77 K. We estimate the corresponding values of the intrinsic R/sub s/ at 7.95 GHz to be 23 /spl mu//spl Omega/ and 110 /spl mu//spl Omega/ at 15 K and 77 K, respectively. These values are comparable with those of other high quality unpatterned YBCO films reported in the literature. Z/sub s/ for the best optimised films appears to be insensitive to the effects of patterning. >

Power-induced switching in YBa 2Cu 3O 7 microwave coplanar resonators

We have studied the power-induced switching of a number of thin film YBa 2Cu 3O 7 (YBCO) coplanar transmission line resonators at 8GHz. Switching occurs at current densities around 10 6 − 10 7 Acm -2 at low T, where it is hysteretic; at higher T it becomes non-hysteretic, and washes out in a narrow temperature range which we believe corresponds to the critical temperature T cj of a single responsible weak link. We develop a thermal model which accounts for all of the observed features of the switching.

Monolithic Y-Ba-Cu-O structures fabricated using the laser-writing patterning technique

We report our progress in fabrication of thin-film YBa2Cu3Ox (YBCO) superconducting electronic devices, using a recently developed, laser-writing patterning technique. Laser writing allows one to form in the same YBCO film planar patterns that consist of both the oxygen-rich (superconducting) and the oxygen-poor (semiconducting) phases. The patterns are highly uniform with very sharp (less than 1 mu m wide) superconductor-semiconductor interfaces. The oxygen-rich regions possess excellent superconducting properties with critical temperatures as high as those of the best epitaxial films, and critical current densities above 2*106 A cm-2 at 77 K. Simultaneously, the oxygen-poor regions exhibit a disordered-semiconductor-like, thermally activated transport. Below 100 K, they are almost insulating and characterized by relatively low (below 20) dielectric constant and low microwave loss. A number of test structures, consisting of oxygen-rich and oxygen-poor microbridges and coplanar microwave transmission lines and resonators, was fabricated and tested. All these devices are completely monolithic and were used to study DC and microwave transport properties of the oxygen-rich and oxygen-poor YBCO phases.

Microwave surface impedance of patterned YBa 2Cu 3O 7 thin films

We have used microwave coplanar resonators to measure the surface impedance below T c of very high quality patterned thin films of YBa 2Cu 3O 7 on MgO substrates. We obtain surface resistances at 8GHz of 23μΩ and 105μΩ at 15K and 77K, respectively. Our technique allows us to estimate the absolute values of the ab-plane penetration depth γ L( T) and we obtain a value of γ L(0) for the best film around 150nm. At low T we find that γ L(T)/ γ L(0) = 1 + α( T/ T c ) 2 , with α≃0.65 for all of our films. This behaviour differs from that of conventional s-wave superconductors.